The present invention relates generally to fuel burners and, more particularly, to pulverized coal fuel burners having burner nozzle coal diffusers or similar structures for efficiently breaking up, deflecting and dispersing pulverized coal fuel ropes that naturally occur within the burner piping.
Fossil fired steam generators are used in the utility power generation field to generate electricity. It is known that satisfactory combustion of pulverized coal fuel in such steam generators requires a higher percentage of excess air than other fuels such as gas or oil. One reason is the inherent maldistribution of the fuel provided to the combustion furnace of the steam generator, not only between individual burner pipes but also within and at the outlet of the burner discharge nozzles. Normally, complete combustion of a pulverized fuel such as coal requires at least 15% excess air. A significant amount of fan power is required just to provide this excess air. Enhanced fuel distribution could reduce excess air levels, with a concomitant reduction in fan power, so long as flame stability and emissions requirements are met.
Significant power consumption is also involved in overcoming the various pressure losses associated with pneumatic transport of the pulverized coal fuel within the fuel piping and burner nozzles. These pressure losses represent a significant operating cost, both at the stage when competitive bids are compared and evaluated, and also during subsequent plant operation. For this reason, it is also advantageous to reduce these pressure losses as much as possible, which will again reduce primary air fan power requirements.
One of the main causes of large pressure losses in the burner nozzle is related to the dispersion of fuel roping. Fuel roping is the concentration of a pulverized fuel such as coal in a relatively small area of the fuel transport pipe and burner nozzle, and is caused by the centrifugal flow patterns established by elbows and pipe bends in the fuel transport pipe and burner nozzle. The term "fuel roping" is used because the stream of coal transported takes the form of a thick, defined collection of coal particles that visually resembles a rope. Since the fuel transport pipe always makes a transition from a substantially vertical pipe run to a horizontal pipe run at the burners where the fuel is discharged into the furnace for combustion, fuel roping is generally unavoidable.
In the past, some pulverized fuel transport pipes and burner nozzles included a Venturi section which was meant to break up fuel roping and evenly disperse the pulverized fuel at an outlet end of the burner nozzle. U.S. Pat. No. 3,788,796 to Krippene et al., assigned to The Babcock & Wilcox Company, shows such a pulverized fuel burner including a Venturi section and a conical end-shaped rod member. The purpose of this combination was to vary the velocity of the coal-air mixture and to enhance the fuel-air distribution. This particular design was ineffective in reducing fuel roping and the pressure drop through the nozzle.
An improved fuel burner of the type disclosed in U.S. Pat. No. 3,788,796 is provided by U.S. Pat. No. 4,380,202 to LaRue et al., also assigned to The Babcock & Wilcox Company. Disclosed therein is a fuel burner apparatus for a vapor generating unit including a tubular nozzle which is concentrically disposed about the central axis of the burner. The inlet end of the nozzle is flow connected to an elbow pipe, and the nozzle conveys air entrained pulverized fuel for discharge into the combustion chamber of the vapor generating unit. A deflector shaped similar to the upper half of a frustoconical form is mounted on the top half of and angled downward from the inlet end of the tubular nozzle. The deflector creates a converging section within the nozzle. A diffuser having an oblong-diamond plug with ascending and descending sections and a shroud member is located within the nozzle. The cylindrical shroud is mounted to the inside of the tubular nozzle. The nozzle and shroud cooperate to form an outer annular fuel and air flow passageway therebetween, while the shroud and the plug cooperate to form a central annular fuel and air flow passageway therebetween. The central annular fuel and air flow-passageway has a converging inlet and a diverging outlet section. Support means are provided to support and position the diffuser shroud co-axially with the diffuser plug such that the diffuser shroud encircles the diffuser plug.
U.S. Pat. No. 4,380,202 to LaRue et al. is instructive for its discussion of various factors affecting burner nozzle pressure drop. As discussed therein, four main forces contribute to the pressure drop that occurs during the pneumatic conveying of the primary air and pulverized solids in the burner nozzle:
(1) The friction of the fluid against the pipe wall; PA1 (2) The inertia force acting on the fluid; PA1 (3) The inertia and gravity forces acting on the solids; and PA1 (4) The aerodynamic drag force acting on the solids.
In addition, areas of flow separation in the burner nozzle can also lead to pressure losses.
When fuel roping occurs, air flow distribution has a secondary effect on particle distribution. Once a particle attains momentum in a certain direction, it will change its direction of travel primarily by being impacted with a solid surface. Therefore, drag forces between the air and solid particles are of secondary importance, while the momentum (mass) of the particle is of primary importance. It is apparent from the foregoing that a reduction in the pressure drop through the burner nozzle can be accomplished by a reduction in any of the forces that contribute to the pressure drop and an elimination of flow separation. However, any attempt to reduce pressure losses must ensure adequate air-fuel mixing in order to provide flame stability and to meet applicable low NO.sub.x standards.
The aforementioned burners disclosed in the '796 and '202 patents are commonly referred to as dual register burners because they employ two sets of air registers or dampers to control admission of the secondary air (the balance of the air necessary for combustion and which is not used to transport the pulverized fuel) into the furnace. The dual register burner has been the subject of continued development, and finds its most recent embodiment in a design known as the DRB-XCL.RTM. burner, a registered trademark of The Babcock & Wilcox Company. The DRB-XCL.RTM. burner employs, inter alia, air and fuel staging technology, as well as various enhancements for secondary air control and measurement, together with the aforementioned conical diffuser and deflector within the burner nozzle.
In the drawings forming a part of this disclosure, like numerals designate the same or similar elements throughout the several drawings. FIG. 1 discloses a cross-sectional side view of such a dual register burner. As shown therein, the dual register burner 10 is comprised of a coal nozzle 12 which extends through a furnace windbox 14 inbetween a windbox cover plate 16 and a furnace wall 18. A mixture of primary air and pulverized coal 20 is provided to a coal inlet 22 to a burner elbow 24. The mixture 20 of primary air and pulverized coal is transported down along the coal nozzle 12, past a deflector plate 26 and a conical diffuser 28, towards an outlet end 30 of the dual register burner 10. A flame stabilizing ring 32 is provided at the outlet end 30 of the dual register burner 10, and combustion of the fuel and air takes place in furnace combustion chamber 34. Secondary combustion air 36 is provided to the furnace windbox 14 by fan means (not shown) and the amount of such secondary air 36 admitted to any given dual register burner 10 is controlled by means of a sliding air damper 38 and its associated damper drive 40. Total secondary combustion air 36 flow into the dual register burner 10 is measured by air measuring device 42, typically an arrangement of calibrated impact/suction probes, also called air flow monitors (AFM). Just prior to the outlet end 30 of the dual register burner 10, the secondary combustion air 36 is divided into two portions which are conveyed to the outlet end of the burner along inner and outer annular passageways 44 and 46 which encircle the coal nozzle 12. Accordingly, the portion of the secondary air conveyed through inner annular passageway 44 (which is closest to and which encircles the coal nozzle 12) is referred to as inner secondary air, while that portion of the secondary air which is conveyed through the outer annular passageway 46 (which encircles the inner annular passageway 44) is referred to as outer secondary air. Located within outer annular passageway 46 are fixed spin vanes 48 and adjustable spin vanes 50 used to impart a desired spin or swirl into the exiting secondary air 36 as it leaves the dual register burner 10. Fixed spin vanes 48 are only used in the outer annular passageway 46. Inner annular passageway 44 uses both fixed and adjustable spin vanes 48, 50. The position of adjustable spin vanes 50 can be varied by means of drive 52. The secondary air 36 exiting from the dual register burner 10 is also affected by the provision of an air separation plate 54 provided at the outlet end 30 of the dual register burner 10.
Another type of burner, traditionally referred to as a circular burner, predates these dual register burner types and was one of the earliest swirl-stabilized pulverized fuel burners, having been used for more than six decades. The circular burner differs from the dual register burner in two main respects. First, circular burners employ a single air register or damper to admit the secondary air. Second, circular burners firing pulverized fuel typically employ an impeller, located near and axially adjustable with respect to an outlet tip of the burner nozzle, which is used to disperse the primary air and pulverized fuel into the secondary air.
Various circular burner arrangements have been developed. One such arrangement is known in the industry as the cell burner, wherein two (and sometimes three) circular burners are combined in a vertically stacked assembly that operates as a single unit. FIG. 2 shows a two-high cell burner arrangement. The mixture of primary air and pulverized coal 20 is provided to the furnace combustion zone via burner elbow 24, burner nozzle 12 and impeller 58 located at an outlet end 60 of the burner nozzle 12. Secondary air 36 from the windbox 14 is provided to a single adjustable register assembly 61 for each burner nozzle 12. The register assemblies are adjusted by drive means 52, and the impeller 58 is located axially with respect to the outlet end 60 of the burner nozzle 12 by means of a shaft 62.
The cell burner has also undergone significant improvements over the last few years, and finds its most recent embodiment in a design known as the LNCB.RTM. burner, a registered trademark of The Babcock & Wilcox Company, (and also known as the Low NO.sub.x Cell.TM. burner, a trademark of The Babcock & Wilcox Company) which was developed in cooperation with the Electric Power Research Institute to achieve reduced NO.sub.x emissions. As shown in FIGS. 3 and 4, this burner modifies a conventional two-high cell burner to supply all of the pulverized coal fuel to the lower burner throat along with a portion of the secondary air. The upper cell burner nozzle is then converted into an integral NO.sub.x port which supplies the balance of the secondary air at each location. The lower burner of each cell is a circular burner, and is provided with the aforementioned impeller 58 at the outer tip of the burner nozzle 12, and may be provided with either the aforementioned coal deflector 26 and conical diffuser 28 described in connection with the dual register burner 10, supra, or alternatively with a convergent distribution cone 66 upstream thereof, located as described below.
FIG. 5 shows a cross-sectional side view through just the burner nozzle 12 of a conventional burner manufactured by The Babcock & Wilcox Company. The burner comprises burner nozzle 12 and elbow 24 to convey the mixture of primary air and pulverized coal 20 to the outlet end 30 of the burner. Impeller 58, typically conical in configuration (some designs have employed bladed-type impellers) is located at the outlet end 30 of the burner nozzle 12. The impeller 58 is axially adjustable with respect to the outlet end 60 of the burner nozzle 12 by means of shaft 62, which may be supported by support means 64 at an intermediate location along the burner nozzle 12, or by a foot as shown in FIGS. 2 and 3, and disperses the mixture of primary air and pulverized coal 20. As indicated in the immediately preceding paragraph, some circular burners manufactured by The Babcock & Wilcox Company have employed a convergent distribution cone 66 in combination with and upstream of the impeller 58 to provide a desired fuel distribution entering the impeller 58. The impeller 58 is used to disperse the coal into the secondary combustion air to a desired degree and thereby affect flame shape. The convergent distribution cone 66 takes the shape of one half of a frustoconical cone, fixed within the burner nozzle 12 at an inner wall thereof, and is positioned 1.5 nozzle diameters (1.5 D) downstream of and centered at the outer tangential centerline of the burner elbow 24 attached at an inlet end 68 of the burner nozzle 12.
Visual observations (schematically depicted in FIG. 6) made during scaled flow model testing of a burner nozzle 12 employing the aforementioned coal impeller 58 and convergent distribution cone 66 reveal that a rope, depicted as a solid area 70, of particles flowing therethrough tends to oscillate within the burner nozzle 12 and bypass the convergent distribution cone 66 positioned 1.5 diameters downstream of the burner elbow 24. Thus the convergent distribution cone 66 did not break up and bias the rope 70 to the outside walls of the burner nozzle 12 prior to entering the impeller 58.
Further testing was performed to determine the design changes necessary to efficiently break up and disperse a rope of particles within the burner nozzle while improving upon the pressure drop characteristics of the burner nozzle. The results of this testing led to the subject matter of the present invention.